We are losing our languages. Today around 6,000 are spoken. By the end of the century half those will be gone. That's one every two weeks. In his forceful and humane book, Mark Abley listens to some of the world's endangered languages, wondering what it means 'to feel the language of [one's] childhood melting away'.

He meets Patrick Nudjulu, the lonely speaker of Mati Ke, an Aboriginal language that may never have had more than 1,000 speakers - all it needed to survive - but is now almost extinct. Nudjulu's sister speaks it too, but not to him; tribal taboos forbid them to communicate after puberty. Of many such elegiac moments in Spoken Here, perhaps most poignant is the story of a parrot speaking a language that had died out among humans and that its keepers could not comprehend.

Meanwhile, English gets more robust and widespread every day. Four hundred million people speak it as a mother tongue; more than one and a half billion speak it as a foreign language. Some are willing to do anything to acquire it. Abley reports, appalled, that wealthy South Koreans take their toddlers to plastic surgeons for frenectomies, tongue-lengthening operations intended to help them pronounce English. Maybe English is the real Esperanto; a vital, complex language that everyone can learn. Maybe it's a waste of time to learn languages that travel badly. Why preserve them past their sell-by dates?

Abley finds many reasons to keep languages. Who would want to lose Mi'kmaq, which with its complex verb phrases and scant nouns posits a reality not fixed but in perpetual oscillation? Abley also relishes Hixkaryana, a Brazilian language which reverses sentence order to put the object first and the subject last; the sentence toto yahosIye kamara (man grabbed jaguar') does not describe crazy heroism; it should translate to 'jaguar grabbed man'. They think we talk backwards.

Encountering the south-east Asian language Boro, with words meaning 'to love for the last time' (onsra) and 'to pretend to love (onsay), Abley falls head over heels: 'Having met those words... how can I do without them? I covet them... They are more than just fresh sounds on the tongue; they are fresh thoughts in the mind.' Other Boro words yield still more pleasure: 'How could anyone resist a language whose expression for "slightly humpbacked" is gobdobdob?'

It's about more than fun with dictionaries, though; language is political. Aborigines planning land-claim have a stronger case if they can prove their connection to the land, often by linguistic associations; knowing the name of a waterhole can be crucial. Native American languages have been lost because children were punished for speaking them at school.

One that clings to life is Yuchi - an isolate, a language that bears no relation to any other living tongue - spoken mainly now by elders at meals hosted by an anthropologist. They speak for their supper and he records a language on the brink. 'When the language goes,' one elder tells Abley, 'the government will be able to say there's no such tribe as the Yuchi.'

Closer to home, Margaret Thatcher wooed the Welsh by promising a TV channel dedicated to their language. Once in power she reneged, but caved in after Gwynfor Evans, president of Plaid Cymru, threatened a hunger strike; the result was S4C.

Perhaps you have to be a fanatic to keep a language alive. Eliezer Ben-Yehuda, the uncompromising inventor of modern Hebrew, refused to speak to his wife in any other language, despite the fact that she could not understand it. He made sure the family pets were of different sexes so that their children would grasp the rules of Hebrew gender. He coined new words to modernise the ancient language and to fit it for secular use.

Now it is Yiddish that is at risk; one of Abley's more quixotic interviewees is a rabbi who thinks that if it had been chosen as Israel's national language, the country might be a softer, gentler place. For poet Chava Rosenfarb, writing in Yiddish is an act of 'fidelity to a vanished language, as if to prove that Nazism did not succeed in extinguishing that language's last breath'.

One reason the Zionists chose Hebrew was that it was common to all Jews, not exclusive of Jews who spoke their own diaspora languages such as Ladino, Judeo-Persian and Judeo-Arabic. All are now dying. Abley believes that linguistic diversity should be taken as seriously as biodiversity. He argues fervently, and convincingly, that the battle to save languages may even 'be part of a wider war, perhaps the central one of our time: the fight to sustain diversity on a planet where globalising, assimilating and eradicating occur on a massive scale.'